My father died 7 years ago after my parents had been married for more than 50 years. After a few years of solitude, my mother started to develop a whole new life (with 80). I think this will develop also for your father, it just takes time. But he has to be ready for it and there is little you can do to accelerate the process. Maybe he was busy with you and your sister until recently?
Key for him is to leave the house. Go to the doctor, buy fresh food on the market instead of in the mall and chat a few words with the farmers, join sports teams (my mother did when she was 78) or meet with colleagues for a beer (they all want an excuse not to go home, believe me).
My brother and I also live far away. That is not a problem (as long as he is in good health…), because it assures that your father has to manage his life for himself instead of sitting down and let you organize it. Also make him visit you instead of the other way round. This will let him leave the house, see different towns, revive his interest in life and discovery and he has to make plans (this is important). It worked for us, at least.
Where I live, also the church has groups for tea/coffee and cake. So has the red cross. This may aim at older people but it is great to go there. My mother loves it. (yes, I know, she is a woman and women make contacts easier).
But, as I said, some day he will be ready.